In Switzerland, gravitational waves go a long way. Not only were they predicted by Albert Einstein in his famous general theory of relativity; Swiss researchers have been involved in attempts to detect gravitational waves from the very beginning and are keen to lend their expertise, including from particle physics, to future projects as well. Here’s an update about the current state of research…
Shubhanshu Tiwari was in the first year of doing his PhD at the Gran Sasso Science Institute in Italy when the first ever signal of a gravitational wave was picked up by the LIGO detector. “My supervisor was the one who saw the signal first. It was a beautiful event”, he remembers. “I was incredibly lucky to be there at that time.” That was in September 2015, and the next weeks are a haze for Tiwari, now a researcher at the University of Zurich. “We had weeks of sleepless nights,” he says, nights spent checking data to make sure it really was the signal the researchers had been looking for.
It did turn out to be real: the first directly detected signal of a gravitational wave, generated when two black holes collapsed into each other some 1.3 billion lightyears away. The wave travelled through space (and time), contorting it along the way, including the arms of the LIGO experiment in the US, causing a signal in the detector.
As Einstein predicted, gravitational waves are produced when big things happen in the vast expanse of space: collisions of black holes or neutron stars, or supernovae, which are stars exploding at the end of their lives. These big events disrupt spacetime, sending ripples in all directions that carry information about their origins and possibly about the nature of gravity itself. Researchers just need the right tools to read this information.
Telescopes had been used since the 1970s to find indirect clues of the existence of gravitational waves, but the first experiments for their direct detection only got underway in the 1990s with earth-based LIGO and prototypes of the planned space-borne LISA gravitational wave observatory. LISA will be a set of satellites equipped with highly sensitive interferometers and an arm length a million times larger than those on Earth, meaning it will be able to pick up a completely different range of signals. It has yet to be approved, but the first step towards the international LISA mission was LISA Pathfinder, which ended in 2017.
LISA Pathfinder was devised to prove that LISA can not only do the science but also deliver the technology, which is where Swiss researchers came in with two professors: physicist Philippe Jetzer, from the University of Zurich, and geologist and seismologist Domenico Giardini, from ETH Zurich. They were able to put science and technology together, and Swiss industry is earmarked to play a major role in the final mission as well. “It is anticipated that every laser that will go to space in the LISA mission will have been calibrated by the R&D organisation CSEM based in Neuchatel,” Steven Schramm from the University of Geneva explains. “NASA will build the lasers and ship them to Switzerland just for the purpose of metrology and calibration, i.e. a very thorough programme of quality checks and controls.”
Schramm, a particle physicist by training, is part of another future project to learn more about gravitational waves and thus the evolution of our universe: the so-called Einstein Telescope (ET). Its concept is similar to those of the existing gravitational wave experiments LIGO, VIRGO, KAGRA and GEO 600 with long arms, lasers and precise interferometry. However, ET would be built in a triangle, its arms would be 10 kilometres long and it would be underground and cooled down to very low temperatures to increase the sensitivity.
The Einstein Telescope poses completely new technological challenges, one of which is dealing with the dramatic increase in the number of gravitational waves that will be observed. In order to pull together expertise from theory, astronomy and particle physics and tackle these challenges, the University of Geneva recently founded the interdisciplinary “Gravitational Wave Science Centre.” “Sorting through noise, making sure signals are recognised as such, recording data in the order of one signal per second – all that isn’t a such big deal when you come from a LHC computing environment,” says Schramm, a computing expert on the ATLAS experiment. “The tricky thing here is to combine all of the measurements and process the data both quickly and accurately, and then immediately distribute the result to external parties. It’s a complex calculation requiring very fast throughput and collaboration; so overall an interesting challenge.”
The goal is to be able to alert research partners – other kinds of telescopes and observatories around the world – immediately when exciting signals come in so that they can point their detectors into the right direction and record the signal in their respective wavelengths. This concept, called multi-messenger astronomy, promises to provide the most complete picture yet of the rules and laws in the Universe.
Since the first direct detection seven years ago, many more gravitational waves from different galactic sources and events have been recorded and studied. The University of Zurich has been part of the LIGO Scientific Collaboration for five years and is not only the first, but also the only Swiss institute that contributes directly to the gravitational-wave detectors currently in operation on the ground, namely LIGO (US), VIRGO (Italy) and KAGRA (Japan). Its members have been very active in the collaboration, leading observational efforts within it, managing R&D groups or spearheading collaboration papers.
Maria Haney is an astrophysicist in the UZH group. She wants to make sure everything is caught, even signals from unusual sources, by developing theoretical models for gravitational waves from black-hole and neutron-star binaries and data analysis tools for the astrophysical interpretation of these sources. She has been dividing her research time between several gravitational-wave projects, and while she currently is preparing for the next observation period of the LIGO-Virgo-KAGRA instruments, she will join the Dutch national laboratory Nikhef in the summer, where she will focus on theoretical and data analysis development for the Einstein Telescope. In fact her work is relevant to all of these current and planned gravitational-wave projects.
Maria Haney and Shubhanshu Tiwari are putting their expertise in theoretical signal predictions and data analysis development together to also find gravitational waves from those kinds of events that have not received much attention so far and might thus be put off brushed aside as noise or misinterpreted. So far, the experiments have picked up waves from rotating binary star systems that are merging along circular orbits. What if these orbits are eccentric? “Such signals might be rare, but they would look very different in the detector data,” Haney says. In a dense environment like a galaxy cluster, black hole binaries could be formed at such short distances that they wouldn’t be able to form a circular orbit because of their gravitational pull. “These systems are very interesting because they can tell us more about the way black holes work,” Haney explains.
While the detectors currently in operation are constantly recording interesting events, the planned future projects will be able to see not only a wider variety of waves because of their different configurations, but also record much more of them. If they get approval, the Einstein Telescope would start around 2035, LISA around 2037. The decisions are expected in three to four years, and whatever happens, Swiss scientists are sure to play a major role.
Swiss Institute of Particle Physics (CHIPP)
c/o Prof. Dr. Michele Weber
University of Bern
Laboratory for High Energy Physics LHEP